Electronic time stamp or time measurement circuits are used to produce highly precise digital time measurements for a wide variety of applications such as automated test equipment, bench top time measurement equipment, radar and sonar devices, etc. Conventional time stamp circuits generate the time stamps using a digital counter that increments at each rising edge of an oscillating clock signal, with the resulting count from the digital counter representing the time at which an event occurred. For example, the time it takes for a radar signal to travel to a target, reflect and return can be measured using two time stamps, one taken when the radar signal is transmitted and another taken when the reflected radar signal returns. The counter values for the two time stamps may be subtracted to calculate the elapsed time.
The precision at which a time stamp circuit can measure the time between two events is typically dependent on the clock speed. Generating precise time stamps for high speed events thus becomes complicated by the difficulty in generating high speed clocks and electronic circuits that can count the high speed clocks. For example, generating a time stamp with a resolution of 833 picoseconds requires a clock that runs at 1.2 GHz. If a time stamp with a resolution of 13 ps is required, the clock in a traditional time stamp circuit must run at 76.9 GHz. Thus, as the resolution required for a time stamp increases, it becomes impossible for conventional electronic time stamp circuits to run and count quickly enough to provide the required resolution.
Hence, for at least the aforementioned reasons, there exists a need in the art for a time stamp apparatus having a resolution that is not limited by the clock speed.